1. Field of the Invention
This invention relates to a color picture tube with a magnetic focusing device which provides an improved circularity of an electron beam spot.
2. Cross-Reference to Related Application
In a focusing device for focusing electron beams in a color picture tube with electron guns for red, blue and green arranged in a row, which is disclosed in U.S. application Ser. No. 917,179 filed by one of the present applicants on June 20, 1978 and assigned to the same assignee, a color picture tube is provided at the neck portion with a couple of magnetic members having through-holes permitting the electron beams to pass therethrough and disposed separately in the tube axial direction, and a magnetizing means for magnetizing the respective magnetic members, thereby to form focusing magnetic lenses in spaces between pairs of through-holes in which the paired holes are opposite to each other.
The magnetic focusing device will further be described with reference to FIG. 1 and FIGS. 2a and 2b. FIG. 1 shows a longitudinal sectional view of an inline type three-gun color picture tube with the magnetic focusing device, FIG. 2a shows an enlarged sectional view of a neck portion of the tube shown in FIG. 1, and FIG. 2b shows an enlarged cross sectional view taken along line IIa--IIb of FIG. 1. In the figures, reference numerals 1, 2 and 3 designate electron guns for red, green and blue for emitting red, green and blue electron beams 91 to 93, respectively; 4 a glass tube portion of a cathode ray tube; 5 a tubular permanent magnet disposed around the glass tube portion 4; 6 and 7 magnetic members of high permeability with through-holes 61 to 63 and 71 to 73 through which the electron beams 91, 92 and 93 pass; 8 a fluorescent screen. A magnetic flux emanating from the N pole of the tubular permanent magnet 5 enters the tubular part 74 of the magnetic member 7, emanates from the holes 71, 72 and 73 toward the holes 61, 62 and 63 of the magnetic member 6, enters again the magnetic member 6, emanating from the tubular part 64 of the magnetic member 6, and returns to the S pole of the tubular permanent magnet 5. Therefore, the periphery of each of holes 71 to 73 is magnetized in an S pole while the periphery of each of holes 61 to 63 is magnetized in an N pole. As a result, focusing magnetic fields are formed between the respective pairs of the holes 61 and 71, 62 and 72, and 63 and 73, respectively.
With this magnetic focusing device, the magnetic field component in the direction (X direction) in which the electron guns are arranged, is weaker than that in the direction (Y direction) orthogonal to the X direction. Therefore, the focusing magnetic lenses formed are not symmetrical with respect to the tube axis. In the case of the focusing magnetic lens for the central electron beam 92, the magnetic flux in the X direction which passes through the magnetic member 7 to reach the hole 72 is partly emitted from the adjacent holes 71 and 73, with the result that the magnetic field in the X direction is weaker than that in the Y direction, by about 10% which is confirmed by our experiment. Accordingly, the focusings of the electron beams 91, 92 and 93 in the X direction are different from those in the Y direction, so that the beam spots of those beams are each not circular and are flatly deformed in the Y direction to have an elliptical shape in the X direction. When the electron beam passes through the focusing magnetic field, there is generated a rotational force rotating about its center of the beam traveling path. By the rotational force generated, the beam spot flatly deformed rotates and then impinges upon the fluorescent screen.
When the spot is flatly deformed, the resolution in the widened direction of the beam spot is considerably deteriorated and further the adjacent focusings are adversely affected to a great extent.